Dennis Mc­Carthy, a car coordinator whose work appears in The Green Hornet, has built stunt and race cars for 20 years. "Beware the first-time roll-cage ­builder," he says.

A roll cage is a specially-constructed tubular frame built in or around the cab of a vehicle to protect its occupants from injury in the case of an accident or rollover. A standard feature in race cars and stunt cars, roll cages are made of steel tubing and include a geometric design to enhance stability and strength. "Beware the first time roll cage builder," warns Dennis McCarthy, a veteran Hollywood stunt and race car builder whose car coordinator credits include The Fast and the Furious, The Green Hornet and Batman Begins. "Always consult with someone who knows what they're doing. And hopefully you'll never have to test your work, he says.

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"A simple four-point cage can be done in a day," McCarthy says. By "four point" he means a cage that anchors to the floor or frame of the car at four places. A "six-point" cage, as the drawing to the left shows, has six anchor points. An extensive stunt car cage can take up to a week. (These times assume some skill in welding and tube cutting.)

Tools:

- 1 3/4-inch DOM mild steel tubing

- Tube bender

- Tube notcher

- Tape measure

- Metal pipe cutter

- Brace

- Hand grinder

- Protractor or carpenter's angle finder

- MIG or TIG welder

Step-By-Step:

1. Make Room

You need to remove the seats and carpet from the vehicle, so you can access the bare-metal floor. Also carpets and seats get in the way and pose a fire hazard. You may choose to remove the headliner as well, but it is nearly impossible to replace it after you've installed the roll bar.

2. Size Up the Ride

Measure the interior dimensions of the vehicle to determine how much tubing you'll need. Measure the distance from the floor to the ceiling, from the rear of the seats to the front firewall, and the width of the interior. Before you order the tubing, consider the width and thickness of the material. If the car will be used for racing, most sanctioning bodies dictate the dimension based on vehicle weight. A good starting point is 1 3/4-in round tube with a 0.120-inch wall thickness.

3. The Base Plates

Arguably the hardest part will be welding steel mounting plates to the floor of the car. The legs of the cage will attach to these plates (in this case we'll have four) so it's critical to get them right. You want to use large base plates (generally 36 square inches and 1/8-inch thick) to spread the load. The placement of these plates is up to you, but generally, you want the rear ones behind the front seats on either side of the floor. The front ones will typically affix to the floor at the junction of the front firewall.

4. Contour the Base Plates

Car floors are typically ribbed to provide strength, so you'll have to bend the base plates to match this shape. You don't want any gaps between the base plates and the floor. There are a variety of techniques for shaping the stiff steel plates. You can simply hammer them over an anvil or vise, or apply some heat if necessary.

5. Weld in the Base Plates

We're taking time on these plates, because they're important. Be careful when you weld because the base plate will be much thicker than the sheetmetal you're welding too—it's all too easy to burn through the car's floor. If you're new to welding, we suggest finding an expert for this step.

6. Bend the Main Hoop

The main hoop is a continuous piece of tube that's bent into a U shape and is welded to rear base plates. Generally, you're looking at two 90-degree bends near the car's ceiling. Once you've bent and cut it to fit—we suggest making a little long to start, you can always trim— tack it to the rear base plates. Make sure it's plumb and square to the car.

7. Take Rough Measurements for Your Supporting Bars

There are lots of options for these supporting bars. You could simply use straight tube that travels diagonally from the top of the main hoop to those front base plates (remember, there's one on each side of the car). A better option is a curved pipe, that travels forward from the main hoop along the car's ceiling and then follows the A pillar to the floor. Obviously each will have a different length. Often we use lengths of string to get a rough estimate of the distance. Again, err on the long side and trim to fit.

8. "Fish Mouth" One End of the Supporting Bars

One end of the supporting bars will anchor to the front base plates. The other will be welded to the main hoop. To make this joint, use either tubing notcher or a grinder to contour the end of the bar so it fits the main hoop. This will soak up most of your time, but get it right. Tack weld the supporting bars.

9. Final Welding

When you're sure all the bars are in the right place, finish weld all the joint. Hopefully, you've left enough space between the main hoop and the ceiling to weld all the way around the joints—most sanctioning bodies will inspect for this critical step. If you have welds near the headliner or other interior panels, use an aluminum plate or other non-weldable, non-flammable material to protect the parts you don't want burned.

10. Bracing

Now that the basic skeleton is finished, braces can be added. At a minimum, weld in a straight piece of tube across the inside of the main hoop, to provide an anchor point for shoulder belts (By the way, this bar should be slightly higher than the driver's shoulders). You also might want door bars to protect the driver from side impacts. As before, fish mouth the tubes so they fit tightly.

11. Don't Paint the Cage

"On a race car or stunt car you want to leave the cage unpainted in order to inspect for cracks after a race or wreck," Dennis says. "The exception is Nascar and a few other organizations, but they only run their cars for a few races before they are replaced. Another option is clear coat, which will prevent rust, but still reveal any cracks.

Glossary:

Base Plate: A precisely-ground plate that acts as the foundation of a fixture.

Tacking: Fitting metal parts together with small welds that can be easily snapped apart to check measurements and allow adjustments before the final weld.

TIG Welding: An arc welding process that uses a nonconsumable tungsten electrode to heat and combine the workpiece metal and the filler rod.

MIG Welding: A welding process that uses motor-driven spool of consumable metal wire as a combined electrode and filler material. MIG is generally a much easier weld to learn, but not as precise as a TIG welder.

See an overview of a Monte Carlo McCarthy modified for an installment of the Fast and the Furious series: